Ultrasound scanners provide useful information about the interior characteristics of an object under examination. In medical applications, clinicians have used ultrasound scanners to examine human subjects in settings such as hospitals, physician's offices, and other locations. Ultrasound scanners have been used in the emergency room, operating room, and similar environments.
Spatial compounding is the process of registering and combing ultrasound frames of the same structure that were acquired at different times and different angles of insonation to form a single compounded or combined frame. The individual frames can be obtained by holding the transducer at a fixed position and transmitting the beam at different angles via electronic beam steering and/or electronically controlled mechanical steering of the array of transducer elements inside the scanhead.
To form a single compounded frame, frames shot with different steering angles are aligned and combined. Alignment can be done based purely on geometric transforms using the transducer array geometry constants as well as the applied steering angles, the sampling frequency, and the speed of sound in the insonated tissue. Sources of incorrect alignment include transducer motion, respiration, incorrect speed of sound, varying speed of sound, and refraction of the beam due to interfaces of varying acoustic impedance making the sound wave alter direction. The latter, in particular, is an issue with spatial compounding because the refracted beam direction is a function of the angle with which the insonated beam hits the interface.
To compensate for motion (transducer motion and/or respiration) registration (rigid and/or elastic) of image features visible in the frames to be aligned has been proposed. The limited success for this approach may be due to the problem of finding out what image to align to. As refraction causes a particular image feature to appear in slightly different spatial locations of the image depending of the steering angle, a strong periodicity is introduced in the series of acquired frames. Even (hypothetical) perfect registration and alignment to the latest image would not produce an acceptable series of compounded images, although every single image in the sequence of compounded images would be crisp and clear. Consequently, compensation for motion by registration has so far only been proven successful in practice for non-compounding applications such as cardiac imaging.
Compounded frames generally have lower speckle and better specular reflector delineation relative to the individual frames making up the compounded frames. Generally, speckle is reduced by the square root of N in a compounded frame with N frames, provided that the frames used to create the compounded frame are substantially independent and are averaged. For specular reflector delineation, spatial compound scanning improves frame quality by improving the acquisition of specular interfaces. The final compounded frame generally has improved contrast resolution and enhanced border visualization.
However, as the insonated tissue may be moving due to respiration, heart beat, etc., and as the transducer is purposely or unpurposely (not held still) moving, the features in the frames being combined may not align very well. In general, it may be difficult to know the exact location of a feature in a frame, which has been inferred from crude assumptions on the speed of sound of the insonated tissue. Unfortunately, miss-registration of the frames (the features in the frames) may introduce artifacts into the resulting compounded frame. Such artifacts may include, but are not limited to, blurring, aliasing, and/or duplication of imaged features.